The Impact of Biodiesel-based Na on the Durability of Cu-Zeolite SCR Catalysts and Other Diesel Aftertreatment Devices

Biodiesel fuel has increased in popularity in recent years as an alternative fuel choice, but there are concerns related to the impact it will have on diesel engines and aftertreatment systems relative to conventional diesel fuel. One major concern is the presence of sodium (Na) in finished biodiesel fuel due to the use of Na-hydroxyl as a liquid-phase catalyst during biodiesel synthesis. The current study focuses on determining the impact of biodiesel-based Na on the performance and materials characterization of diesel aftertreatment devices including lean NOx traps (LNT), diesel oxidation catalysts (DOC), diesel particulate filters (DPF), and Cu-zeolite selective catalytic reduction (SCR) catalysts. Long-term engine aged LNT, DOC, and DPF samples are provided by research partners, while a 517 cc single-cylinder Hatz diesel engine is used to perform accelerated Na-aging of aftertreatment systems consisting of a DOC, SCR, and DPF in either the light-duty (DOC-SCR-DPF) or heavy-duty (DOC-DPF-SCR) configuration. Bench-flow reactor (BFR) evaluations reveal that the performance of LNT and DOC catalysts is negligibly affected by exposure to Na, but that Cu-zeolite SCR in the light-duty configuration suffers a drastic reduction in nitrogen oxide (NOx) performance. The performance loss can be avoided by placing the SCR downstream of the DPF in the heavy-duty aftertreatment configuration, but electron microprobe analysis (EPMA) of the DPF from this configuration identifies excessive Na ash buildup and migration of Na into the DPF substrate. v EPMA analysis of the Na-aged SCR determined that the contamination pattern is similar to that observed in the long-term engine-aged DOC and LNT samples, providing credibility to the accelerated Na-aging process. Materials characterization techniques including diffuse-reflective infrared Fourier transform spectroscopy (DRIFTS), scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), and BET surface area measurements determined that loss of catalyst surface area and a decrease in the number of active Cu sites for ammonia (NH₃) adsorption and SCR reactions are the most likely cause of the reduced nitrogen oxides (NO_x) performance in the light-duty configuration accelerated Na-aged SCR. Finally, mathematical modeling successfully predicts the performance of fresh SCR catalysts, but is less accurate for catalysts exposed to elevated levels of Na.